The present invention relates to a silica glass member and projection exposure apparatus. More particularly, the present invention relates to a silica glass member used for optically transparent, optical members such as lenses, prisms, plates, etc. in optical systems of apparatus utilizing a light source emitting ultraviolet light, e.g., an ArF excimer laser or the like, for example, such as projection exposure apparatus, CVD apparatus, laser beam machines, etc., and projection exposure apparatus using the silica glass member.
The silica glass member stated in the present invention embraces optical members (final products) such as lenses, prisms, mirrors, plates, etc. used in the optical systems and also embraces silica glass ingots and members partly cut out thereof (half finished products (materials) for fabrication of the optical members).
The exposure apparatus called a stepper is used in the photolithography technology of exposing and transcribing micropatterns of integrated circuits on a wafer of silicon or the like. The optical system of this stepper is comprised of an illumination optical system for uniformly illuminating a reticle with an integrated circuit pattern thereon, with light from a light source, and a projection optical system for projecting and transcribing the integrated circuit pattern of the reticle at a predetermined reduction ratio (e.g., 1/5) on the wafer.
With recent increase in the degree of integration of LSI, the light sources of the exposure apparatus such as the steppers and the like are proceeding toward shorter wavelengths, from the g-line (436 nm) to the i-line (365 nm) and further to KrF (248.3 nm) and ArF (193.4 nm) excimer lasers. In the case of an example of DRAM among VLSI, as the capacity increased in the order of 1 K, 256 K, 1 M, 4 M, 16 M, 64 M, and 256 M with development from LSI to VLSI, the work line width decreased in the order of 10 xcexcm, 2 xcexcm, 1 xcexcm, 0.8 xcexcm, 0.5 xcexcm, 0.3 xcexcm, and 0.2 xcexcm, and there are increasing demands for the exposure apparatus capable of implementing exposure in smaller minimum work line width. The light source in this case must be one emitting the ultraviolet light or vacuum ultraviolet light under 250 nm, such as the excimer lasers and the like.
In general, optical glasses used for the lens members of the illumination optical system or the projection optical system in the steppers using the light source of a longer wavelength than the i-line, quickly decrease their optical transmittance in the wavelength region shorter than the i-line and most of the optical glasses transmit no light, particularly, in the wavelength region under 250 nm. For this reason, materials that can be used for the optical systems of the steppers with the excimer laser as a light source are limited to silica glass and some crystal materials. Among others, the silica glass is a material that is commonly used in the ordinary optical systems of ultraviolet and vacuum ultraviolet light as well as the excimer laser steppers.
For implementing exposure of the integrated circuit pattern in large area and high resolution in the projection exposure apparatus, a silica glass optical member used in its optical system is required to have high quality. For example, even a very large aperture member approximately 250 mm in diameter is required to have such homogeneity of refractive index distribution that the difference between maximum and minimum of index distribution of the member is of 10xe2x88x926 or less order. For improvement in the resolution of the optical system, it is also important to decrease birefringence, i.e., to decrease internal strain of the optical member, as well as the improvement in the homogeneity of refractive index distribution.
Furthermore, the optical member needs to demonstrate distinctly excellent transmittance, while exhibiting the high quality in the homogeneity of index distribution and the strain as described above. For example, the illumination optical system or the projection optical system of the exposure apparatus needs to be constructed of a considerably large number of lenses and others for correction for aberration, so that the total path length of the entire optical system can be even 1000 mm or more. For keeping the throughput of the optical system, the optical members in use are desired to have the absorption coefficient of not more than 0.003 cmxe2x88x921. Furthermore, each optical member needs to maintain such high transmittance, not only in the central area of the member, but throughout the entire region. For this reason, among the silica glasses solely, only a limited number of silica glasses can be applied to the accurate optical systems such as the excimer laser steppers and the like.
Among the silica glasses, a synthetic silica glass made by a production process called a vapor phase synthesis process is obtained in high purity with a small amount of metal impurities, thus has high transmittance in the ultraviolet region of wavelengths under 250 nm, and further permits a homogeneous silica glass optical member to be formed in large aperture. For this reason, the synthetic silica glass is expected as a promising material for the optical systems of the exposure apparatus such as the excimer laser steppers and the like.
However, even if the exposure apparatus with the light source of the ArF excimer laser or the like was constructed of the lenses and others made of the synthetic silica glass with the excellent characteristics as described above, there occurred variation in illuminance in the exposure area on the wafer and there arose the problem that it was very difficult to implement uniform exposure.
The present invention has been accomplished in view of the problem in the prior art and an object of the present invention is to provide a silica glass member adapted to exposure with ultraviolet light so that it can prevent the absorption of the ultraviolet light and the occurrence of the illuminance variation in the exposure area and implement high-resolution and uniform exposure.
Another object of the present invention is to provide a projection exposure apparatus capable of implementing high-resolution and uniform exposure while preventing the absorption of ultraviolet light in the optical system and reducing the illuminance variation on the reticle surface and in the exposure area on the wafer.
In order to solve the above problem, a silica glass member of the present invention is one wherein when a composition thereof is expressed by SiOx, x is not less than 1.85 nor more than 1.95, wherein a concentration of hydrogen molecules included therein is not less than 1xc3x971016 molecules/cm3 nor more than 5xc3x971018 molecules/cm3, and wherein a difference Axe2x88x92B between an absorption coefficient A immediately before an end of irradiation with 1xc3x97104 pulses of ArF excimer laser light in an average one-pulse energy density of 2 mJ/cm2 and a second absorption coefficient B at 600 seconds after a stop of the irradiation with the ArF excimer laser light is not more than 0.002 cmxe2x88x921.
A projection exposure apparatus of the present invention is one comprising a reticle having a pattern; an illumination optical system for illuminating the reticle with light of a predetermined wavelength; and a projection optical system for focusing the pattern on the reticle illuminated by the illumination optical system, on a wafer, wherein at least either the illumination optical system or the projection optical system comprises a silica glass member wherein when a composition thereof is expressed by SiOx, x is not less than 1.85 nor more than 1.95, wherein a concentration of hydrogen molecules included therein is not less than 1xc3x971016 molecules/cm3 nor more than 5xc3x971018 molecules/cm3, and wherein a difference Axe2x88x92B between an absorption coefficient A immediately before an end of irradiation with 1xc3x97104 pulses of ArF excimer laser light in an average one-pulse energy density of 2 mJ/cm2 and a second absorption coefficient B at ten minutes after a stop of the irradiation with the ArF excimer laser light is not more than 0.002 cmxe2x88x921.
According to the present invention, when the composition, the hydrogen molecule concentration, and the absorption coefficients of the silica glass member satisfy the above specific conditions, the absorption amount is sufficiently decreased, particularly, in low dose regions and variation of absorption coefficient is well restrained in repetitions of ultraviolet irradiation on and irradiation off; it thus becomes feasible to prevent the absorption of ultraviolet light in the silica glass member and the illuminance variation in the exposure area. Accordingly, it becomes feasible to implement high-resolution and uniform exposure when the silica glass member of the present invention is applied to the illumination optical system and/or the projection optical system in the projection exposure apparatus.
In the silica glass member of the present invention, preferably, the absorption coefficient A and the absorption coefficient B satisfy a condition represented by A/B less than 2.
In the silica glass member of the present invention, preferably, an absorption coefficient at a predetermined wavelength in a wavelength region of 190 to 400 nm before the irradiation with the ArF excimer laser light is not more than 0.003 cmxe2x88x921.
Furthermore, in the silica glass member of the present invention, preferably, in a radial distribution of birefringences a difference between a maximum and a minimum of the birefringences is not more than 2.0 nm/cm.
Furthermore, in the silica glass member of the present invention, preferably, a hydroxyl concentration is not less than 800 ppm nor more than 1300 ppm and in a radial distribution of hydroxyl concentrations a difference between a maximum and a minimum of the hydroxyl concentrations is not more than 50 ppm.
Furthermore, in the silica glass member of the present invention, preferably, a content of Na is not more than 20 ppb.